The patient submitted to artificial ventilation is connected to a ventilatory system that most often offers the following modalities of ventilation and respective respiratory cycles. The controlled mandatory ventilation (CMV) where the patient receives respiratory cycles named controlled cycles. The controlled cycles are initiated by the machine, and the patient receives a predetermined volume, at also preset flow and respiratory rate. When receiving controlled cycles during CMV the patient is usually sedated and doesn't present any spontaneous inspiratory effort. Consequently the pressure generated in the system depends on the pulmonary mechanics of the patient, that means, airway resistance and pulmonary compliance, and all respiratory work is performed by the respiratory system apparatus, or ventilator. The other modality is termed Continuous Positive Airway Pressure (CPAP), where the patient still intubated and connected to the ventilator is submitted to a pressure termed positive end expiratory pressure (PEEP) and receives respiratory cycles named spontaneous cycle.
The spontaneous cycles are initiated and maintained exclusively by patient inspiratory effort that generates a negative pressure in the airway transmitted to inside the ventilator, that in response delivers proportional flow and volume. Clinical studies showed that the work of breathing associated to spontaneous cycles during CPAP in intubated patients exceed too much the one in normal subjects, because patient has to breath through tubes and pneumatic valves. An alternative to reduce the work of breathing during spontaneous cycles is the modality termed Pressure Support Ventilation (PSV). In this modality, analogous to the spontaneous cycle, the pressure supported spontaneous cycles are initiated by the patient inspiratory effort. When the patient makes the initial inspiratory effort generating a negative pressure, detected by a trigger system, the ventilator increases the pressure in the respiratory circuit until a predetermined level of Pressure Support (PS). Consequently there will be a demand flow proportional to the difference of intrapulmonary pressure and set PS, and inversely proportional to airway resistance. As volume enters the lung, increasing intrapulmonary pressure depending on compliance, diminishes the pressure difference and so on the flow. When the demand flow decreases to a certain level, usually to 25% of maximum flow achieved during the respiratory cycle, the ventilator returns the pressure level to baseline, allowing the patient to exhale. The rationale for this system is to create a pressure gradient increasing the pressure at ventilator side instead of obliging the patient to decrease intrapulmonary pressure by his own effort. Clinical results showed that PSV has decreased successfully the inspiratory workload of intubated patient. However as it occurs during unsupported spontaneous cycle, during pressure supported spontaneous cycle there is no guarantee of delivered volume or number of delivered cycles, what restricts its use in stable patients with reliable inspiratory drive, being more adequate to the weaning phase of ventilation, prior to extubation. Finally, there are two other modalities, the Assisted Controlled Ventilation (A/CV) and Synchronized Intermittent Mandatory Ventilation (SIMV), that present a third type of respiratory cycle, the assisted cycle. The assisted cycle is similar to a controlled one, that delivers a predetermined volume at also preset flow. Different than the controlled cycles, the assisted ones are initiated by patient effort, like the spontaneous cycle. If patient presents inspiratory effort enough to generate a pressure drop equal to the adjusted by assist sensitivity control, the ventilator initiates the flow and volume delivery triggered by his effort. If after triggering the assisted cycle the patient relax, the assisted cycle will perform exactly as a controlled one. However if after onset of assisted cycle the patient maintains the effort, as the flow is maintained constant in the value and pattern adjusted by the operator, the result will be a decrease in the airway pressure. The conventional assisted cycle is unable to respond in terms of flow to patient effort after initiation of the cycle, resulting only in a airway pressure drop and de synchrony between patient effort and gas demand. This occurrence means that the patient is performing a non controlled amount work of breathing throughout the assisted cycle, when ideally the patient should be unloaded in the immediately post controlled ventilation. When becoming to recover gradually his respiratory function the patient would be submitted to a combination of assisted and spontaneous cycles in SIMV, preferably pressure supported, and then moved to a a modality with spontaneous cycles only, like PSV. Clinical studies showed that in many situations the work of breathing associated to assisted cycles exceed those in spontaneous cycle, when theoretically it should be minimum. This has been reported as the cause of respiratory muscles fatigue, obliging patient sedation and extending ventilation time. The ideal assisted cycle should harmonize both guarantee of volume requirements achieved by the controlled cycles, and the synchronization of patient effort and flow demand achieved by pressure supported spontaneous cycles.